Even if it is (and I think it probably is) an inconsistency which Lewis didn't think of, it's still fun to try to work out some explanation of how it could make sense within the Narnian world.

I would need more time to think of something ...

Oooh, good observations, Erlian. My thought is that it is an inconsistency, but, as Varna mentioned, it would be fun to surmise some kind of explanation, even if Lewis didn't have one. (Tolkien would have. ) I, too, will have to think more on this.

The first answer that leaps to mind is that they never have the same adventure twice.

Second, they were aware they were leaving in PC and SC and in LWW they blunder in and out. LWW is also an anomaly based on the amount of time spent in Narnia, the others are more like a summer holidays worth instead of years and years. In SC and LB there's a bit more attempting to get to Narnia and being prepared for switching worlds.

I don't have a thought on the difference between LWW and PC, but in SC I always thought it was a different transition from Narnia to our world. That is why Aslan and Caspian appear in our world. It seems more like a rip in the separation between the two worlds, rather than a trip between the two.

Every journey to and from Narnia is different and can never be repeated so I guess the question of whether you are wearing the same clothes or have to change once you get back can be different each time too. I love the idea of Jill keeping her outfit and wearing it at a fancy dress party!
Helen (I frequently walk past Lewis' pub the Eagle and Child in Oxford on the way to my favourite French restaurant!)

In LWW, they didn't really intend to return to our world. So when Aslan transformed them back into children, he clothed them too. (He was the one who drew them to Narnia and He was the one that sent them back.)

In PC, Aslan had talked to Peter and Susan. He instructed them to change their clothes because they would be returning home. We don't get to hear the conversation - but since they leave shortly after, it's pretty safe to say that He was told them to change.

In SC, Aslan knew that if Jill and Eustace still had on their Narnia garb, the reaction from the bullies would be different. After all, from the bullies point of view, Jill and Eustace had just run around the corner towards the wall less than a minute before. And the next thing they knew was there were 3 people coming towards them (not 2) dressed in courtier clothes and 2 of them had swords and one had a riding crop.

Aslan is the one in charge in all of the comings and goings. And He never does things the same way twice.

Just like in the Bible, Jesus rarely performs miracles in the exact same way - some He healed with a touch, some He healed just by speaking, and He healed the blind man by making mud from spit and dirt.